In, for example, an automobile equipped with an engine employing natural gas as a fuel, a fuel tank storing compressed natural gas is mounted thereon, and when the engine is operating, high pressure gaseous fuel fed out from the fuel tank is reduced in pressure to a predetermined pressure and supplied to the engine. When compressed natural gas is stored in the fuel tank, lubricating oil within a compressor used for compressing the natural gas contaminates to a small extent the fuel that is compressed, and when gaseous fuel within the fuel tank is fed to the engine as fuel, it is necessary to capture the contaminating liquid from the gaseous fuel. The liquid trap device for a gas is used for capturing contaminating oil from a gas in such a situation, and is already known as disclosed in, for example, Patent Publication 1 below.    Patent Publication 1: Japanese Patent Application Laid-open No. 2007-130558